The present invention relates generally to the field of high speed switching, and more particularly to the field of bistable optical switches.
There is a significant amount of research in the field of bistable optical devices for use in high-speed optical communication systems and optical computers. A bistable optical device is a device that exhibits two distinct states of transmission of light and that can be switched between these two states by a temporary change and in the level of the light input, i.e. the output versus input characteristic exhibits an optical hystersis. Such bistable switches potentially have a variety of attractive features such as (1) an extremely large bandwidth -greater than 10.sup.13 Hz; (2) an ultra-short switching time; (3) a capability for parallel processing; and (4) the ability to process light directly. Accordingly, such bistable switching elements would have significant speed advantages over their electronic component counterparts and can be used in such applications as memory elements, differential amplifiers, pulse shapers and limiters, optical triodes, and logic elements. Note in particular that with the development of glass fiber optical communications systems requiring high-capacity repeater and terminal systems, such bistable devices have the necessary bandwidth and switching speed needed to meet the requirements of such systems.
Currently there are a number of schemes proposed for realizing a bistable optical switch with switching times in the picosecond regime. For example, the review article entitled "Bistable Optical Devices Promise Sub-picosecond Switching" by P. W. Smith and W. J. Tomlinson, IEEE Spectrum, June 1981, page 26, notes that a bistable optical switching device may be formed by disposing a nonlinear optical material within the optical cavity of a Fabry-Perot resonator. The nonlinear optical material is chosen to exhibit a significant optical Kerr effect-that is, the refractive index is a function of the intensity of the light propagating in the material. It has been found that such a device exhibits optical bistability under proper bias conditions. A weak light beam is applied to cause a photo-induced change in the refractive index of the nonlinear material thereby causing a switching to a new transmission state. This switching can occur in picoseconds. However, it has been found that the photon absorption in the material that caused the switching to a new transmission state has a decay which is dominated by carrier recombination time-typically nanoseconds or longer. Thus, this optical switch can not be switched again until after a sufficient time has elapsed for the carriers to recombine. Faster switching could be possible if the nonlinear material disposed in the resonator is highly doped to reduce the carrier lifetime. However, such a high carrier doping would be accompanied by a corresponding decrease in the refractive index nonlinearity which causes the bistability. Accordingly, the carrier recombination process is a major limitation to the switching speed of such a bistable optical device.
Another approach for constructing a bistable optical device is to utilize a Fabry-Perot resonator employing exciton effects. See the articles by H. M. Gibbs, S. L. McCall, and T. N. C. Venkatesan, Optical News 6, Summer (1979); E. Hanamura, Solid State Communications, 38, 939 (1981). The exciton effects in semiconductors are used as the source of the refractive index nonlinearity. However, to date the response times obtained from such devices exceed 10 nanoseconds.
Thus, it can be seen from the above that current schemes for bistable optical switches all have switching times of 1 nanosecond or more.